Safety device for x-ray apparatus



w. K. KEARSLEY 2,058,992-

SAFETY DEVICE FOR X-RAY APPARATUS File d May 28, 1936 "Inventor: William K. Kearsleg, I by His/Attorney.

Patented Oct. 27, 1936 UNITED STATES PATENT OFFICE William K. Kearsley, Schenectady, N. Y., assignor to General Electric Company, a corporation of New York Application May 28, 1930, Serial No. 456,610

9 Claims.

This invention relates to X-ray apparatus and the like and is particularly concerned with means for preventing X-ray tubes from becoming overheated in use.

In present X-ray outfits there is usually no automatic means associated with the X-ray circuit to interrupt the same and deenergize the tube when the temperature of the anode reaches a predetermined excessive value. As a result, overheating of such tubes has been frequent, causing serious loss. While an operator of an X-ray outfit, if he is watchful, may be aware that the tube he is operating is overheating and take steps to deenergize it before it is injured, frequently, as when the outfit is being run uninterruptedly for too long a period of time, the anode may heat up excessively and may even fuse before the operator is aware of what has happened. Again, where the outfit is of the oilimmersed or shock-proof type and the tube is enclosed in a metal tank it is not possible to see the anode and it is hence very easy to overrun the tube and fuse the. anode before the operator knows of the dangerous condition which exists.

In order to provide a safety means for X-ray tubes and the like which will prevent the tube from overheating and which will deenergize the tube by interrupting the circuit whenever a predetermined temperature. of the anode is reached, I have provided a thermostatic switch so related to the X-ray circuit that it will interrupt the latter and deenergize the tube at the proper time. I have furthermore designed the switch and related it to the X-ray circuit so that it operates in a range of temperature far below that of the anode but varies in temperature proportionately to the anode temperature. In other Words, the thermostatic switch is so designed that it heats up in proportion to the heat developed at the anode. While being located in non-thermal relation to the X-ray tube, that is, outside of the sphere of direct heat transfer, it is so related to the X-ray circuit that it is an accurate measure of the energy input to the tube.

My invention will be better understood by reference to the following detailed description thereof taken in connection with the accompanying drawing forming part of the present application and its scope will be pointed out in the appended claims.

In the said drawing, Fig. 1 represents some.- what diagrammatically an X-ray circuit containing one form of my invention; and

Fig. 2 is a detailed diagrammatic showing of conventional X-ray tube of the hot cathode type and have shown my invention as applied thereto.

This X-ray tube I0 includes an anode II which is usually constructed of copper and provided with an anode stern I2. The cathode I3 is con nected as shown with a. filament transformer M which is adapted to provide the electrical energy necessary to heat the filament contained in or forming part of the cathode. A main transformer I5 is provided, as is usual in X-ray circuits of this character, to supply the necessary electromotive force which is impressed across the anode and cathode. Both the main and filament transformers are connected to a source of electromotive force by suitable connection to power supply means such as the main supply line [6 shown in Fig. 1 of the drawing, the connection being made through the medium of an autotransformer l5 which as is well known, permits the voltage impressed upon the transformer IE to be varied.

Between the X-ray outfit thus described and the main supply line is inserted the safety means of the present invention. This safety means comprises a thermostatic switch I! placed in series with the primary winding of the main transformer l5 and is hence a measure of the current supply to the latter. The. safety means also comprises a resistance element l8 in parallel with the primary winding of the main transformer. The thermostatic switch I1 is diagrammatically shown as V-shaped although the actual shape or configuration of this element is immaterial as far as the present invention is concerned. It is connected to the circuit at the ends opposite the apex of the element, the apex being free to move, and is provided with a contactor 19 adapted to make contact with terminals 20 and 2!. The terminal 20 is connected directly to the main supply line while the terminal 2| is connected to the supply line through a solenoid 22. This solenoid is provided with a switch member 23 normally held by means of the spring 24 against terminals 25 and 26 which are suitably connected in series with the pri mary Winding of the transformer IS.

The thermostatic switch is composed of suitable metals and is so proportioned that the rate of heating and cooling thereof corresponds to that of the anode in the X-ray tube. This switch may conveniently be made of a bi-metallic strip consisting of two metals having unequal coefficients of expansion, as for example, invar and brass, which strip when heated causes the contactor l9 thereon to move in the direction of the terminals 20 and 2|.

It will be noted that the thermostatic switch I I being connected in series with the primary of the transformer l5 as shown, is heated in a direct measure of the current input to the latter and hence to the anode of the X-ray tube. On the other hand, the resistance element l8 being con-- nected in shunt with the primary of the trans former l5 heats proportionately to the voltage of the energy supplied to the transformer 55. The shunt resistance I8 is located in close proximity to the switch I! so that the heat developed therein may be transmitted to and cause the metal of the switch to heat up accordingly.

Ordinarily, for the particular type of tube being used, and for the particular purpose to which the tube is being put, the safe energy input is known or may be determined by trial or by calculation. In order that the thermostatic switch may be set so that it will fiex or expand the correct amount to make contact with the terminals 20 and 2|, to open the circuit, adjustable resistance elements 21 and 28 are provided which are respectively connected in shunt with the element I? and in series with the element l8. These reslstance elements may be set at the desired point to insure the correct time characteristic of the thermostatic switch. The shunt resistance It! may properly be termed a heater for as described above, its function is to heat the thermostatic switch l1, its heating effect varying responsive to voltage variations in the supply circuit of the transformer l5. For example, when the X-ray tube is operated at low currents as for instance when used for fiuoroscopy, the current through the thermostatic switch Il may not be sufficient to cause the switch to operate, but the additional heat supplied by the resistance element Hi to the switch [1 will be sufficient to cause the latter to expand sufficiently to make contact with terminals 20 and 2|. The resistance element [8 may be placed either near or far from the switch I! to give the proper time lag. As was pointed out above, the thermostatic switch I! heats proportionately to the value of the current in the circuit. For instance, where a high current is employed, as for example, in radiographers work the switch I I would function to prevent excessive heating of the tube due to the high current. It is thus seen that the safety means which comprises the thermostatic switch I! and resistance element I8 is capable of responding to excess current, or voltage or a combination of the two or when the time during which the X-ray tube is used is so long that the temperature of the anode would tend to attain a predetermined excessive value. It is obvious, however, that if the current and voltage supplied to the tube are below the safe limit of the tube for a given operating interval, then the switch will not function.

In Fig. 2 I have shown a modification of the adjustment for the thermostatic switch. In this case the adjustment is mechanical instead of electrical and comprises spring means 3!) fixed at one end to the free end of the thermostatic switch l1, its other end being fixed to an adjustable element such as the screw 3! which is screwed into a fixed part 32 of the outfit. In this case by manipulating the screw 3! the tension of the spring may be varied to give the desired mechanical resistance to the thermostatic switch 1, thus preventing its making contact with terminals 20 and 2| until the desired flexure or expansion has taken place. The amount of expansion will, of course, be controlled by the predetermined temperature against which it is desired to protect the tube.

The dotted outline 33 surrounding the X-ray tube and transformers, is shown merely to illustrate that my invention is readily applicable to the conventional oil-immersed or shockp1'oof outfit.

In operation, assuming that the switch 29 is closed by the operator, the parts of the circuit are shown in Fig. 1 in their normal position as the tube is operating, the autotransformer 5' having first been set for the proper voltage.

Ordinarily, the electrical adjustments 2! and 28 or the mechanical adjustment 3B-3l whichever happens to be employed in the circuit will be properly set when the switch is first installed so that it will function accurately for the particular tube with which it is associated.

During normal operation of the outfit heat Will be developed at the anode II. The anode H and anode stem 12 are usually constructed of a metal which conducts heat rapidly, such as copper. The safety means being a measure of the wattage input to the anode, will be a measure of the heat developed at the anode; hence if the heat developed at the anode should reach a dangerous limit the thermostatic switch will expand to make contact with the terminals 20 and 2| thereby energizing the solenoid 22 which in turn acts to withdraw the switch member 23 from terminals 25 and 26. This breaks the circuit from the main supply line and the transformer 15 and hence to the X-ray tube. The thermostatic member of the safety device having a heating and cooling characteristic proportional to the heating and cooling characteristic of the anode of the Y-ray tube, it will be seen that when the predetermined temperature of the anode is reached the thermostatic switch will have made contact with the terminals 20 and 21 to interrupt the .X-ray circuit. Similarly, as the anode cools after the circuit has been interrupted and the X-ray tube deenergized, the thermostatic switch and the resistance element l8 also cool, the thermostatic switch due to its design and constmction, cooling at the same rate as the anode. After the switch has sufliciently contracted it breaks contact with the terminals 20 and 2!. The solenoid 22 is thus deenergized, and the spring 24 returns the switch member 23 to its normal position against terminals 25 and 26 whereupon the circuit is automatically closed and the X-ray tube once more brought into operation.

Should it be desired for any reason to increase the current at which the tube is to be operated, or to increase the voltage, as a result of which more energy will be present in the circuit and hence heat is generated at the anode at a higher rate, the thermostatic switch of the present invention due to the construction and arrangement described, will expand proportionately to make contact in less time with the terminals 20 and 2| and thus open the X-ray circuit when the heat so generated causes the anode temperature to approach or reach the predetermined value. The switch will not close the circuit again until the metal of the anode has cooled sufficiently so that it is safe to operate the tube once more. All this is accomplished automatically and without the attention of the operator.

It is to be noted that while the thermostatic switch heats and cools in proportion to the heating and cooling of the anode in the X-ray tube, yet due to its construction its temperature range is far lower than that of the anode when the tube is operating. v For example, while the temperature of the anode may be in the neighborhood of 900 0., the thermostatic switch due to its construction and association in the circuit with the resistance element l8, may, for example, not exceed a temperature of about C. It is evident that this is an important feature of the invention from a practical standpoint.

It will be apparent that many modifications may suggest themselves to those skilled in the art to which this invention pertains but it is to be understood that such modifications are included in the scope of the present invention provided only that they come within the scope of the appended claims.

What I claim as new and desire to secure by Letters Patent of the United States, is,-

1. In combination, an X-ray tube, a power supply means including a transformer, the secondary of which is connected to said X-ray tube, a thermostatic switch connected in series with the primary of said transformer, a resistance heater for said switch connected in parallel with said primary, said switch heating and cooling in proportion to the heating and cooling of an electrode of said tube during operation of said tube and a relay operatively connected to said switch and transformer to deenergize said X-ray tube when the temperature of such electrode reaches a predetermined value.

2. In combination, an X-ray tube, a power supply means therefor including a transformer, a thermostatic switch located out of heat-conducting relation to said X-ray tube, and connected in series with a winding of said transformer, a resistance heater means connected in parallel with said winding, said switch being responsive to current proportionate to the load current of said tube, and said heater means being energized by current proportionate to the voltage of the load current of said tube, said switch heating and cooling in proportion to the heating and cooling of the anode of said tube during the operation thereof, and means in the energy supply circuit responsive to the deflection of said switch when heated, for interrupting the circuit when the temperature of the switch reaches a predetermined value.

3. In combination, an X-ray tube, a power supply means therefor including a transformer, a thermostatic switch connected in series with the primary of said transformer, a resistance means connected in parallel with the primary of said transformer, said switch heating and cooling in proportion to the heating and cooling of the anode of said tube during operation of said tube, and a solenoid switch within said primary circuit connected in circuit with said thermostatic switch, said solenoid switch being connected to interrupt the transformer circuit when the switch moves in one direction in response to a rise of temperature and to restore the circuit when the switch moves in the opposite direction in response to a fall of temperature.

4. In combination, an electrical discharge tube having electrodes one of which may be subject to excessive heating by continued operation of said tube, a power supply means, a thermostatic switch having a movable element connected to be traversed by current proportionate to the load current of said tube, a heater member traversed by current proportionate to the voltage of the load current of said tube, said switch and heater member being located in heat transferring relation to one another but out of heat transferring relation to said tube and means cooperating with said movable member to maintain open the load circuit of said tube only during such times as the temperature of the electrode of said tube is abnormally high.

5. The combination of an X-ray tube having a target, an energy supply circuit for said tube, a thermo-responsive switch having a heating and cooling characteristic proportional to such characteristic of said target, said switch being connected in said circuit and responsive to the current in said circuit, a separate heater for said switch, electrical connections for energizing said heater by current proportionate to the voltage of said supply circuit, said switch being constructed and connected to move from its normal position to a second position when its temperature exceeds a predetermined value, and to return to its normal position when its temperature has been restored to said value, and electrical means connected between said switch and said supply circuit for opening and closing said supply circuit when the switch is operated.

6. In combination, electrical discharge apparatus including a cathode and a cooperating electrode which normally operates at an elevated temperature, an energizing circuit for said apparatus, the temperature of said electrode varying in response to variations of energy supplied to said apparatusa thermostatic switch, means for heating said switch proportionately to the amperage of current supplied to said apparatus, means for heating said switch proportionately to the voltage of current supplied to said apparatus and means controlled by said switch for opening and closing said circuit in response to changes in energy supplied to said apparatus whereby the temperature of said electrode is maintained less than a predetermined excessive temperature.

7. The combination of an X-ray tube including a cathode and an anode, said anode normally operating at an elevated temperature which varies in response to variations of energy supplied to the tube, an energy supply circuit for said tube, a thermo-responsive safety device having a temperature-fiexure characteristic proportional to the heating and cooling characteristic of said anode, means including said energy supply circuit for heating said safety device during operation with energy which varies in proportion to the energy supplied to said tube whereby the temperature of the safety device follows closely the temperature variations of said anode, and means whereby the safety device opens said energy supply circuit when the anode reaches an excessive temperature.

8. The combination of an X-ray tube including a cathode and an anode, said anode normally operating at an elevated temperature which varies in response to variations of energy supplied to the tube, an energy supply circuit for said tube, a thermo-responsive safety device having a temperature-flexure characteristic proportional to the heating and cooling characteristic of said anode, means including said energy supply circuit for heating said safety device during operation with energy which varies in proportion to the energy supplied to said tube whereby the temperature of the safety device follows closely the temperature variations of said anode, and means whereby the safety device opens said energy supply circuit when the anode reaches an excessive temperature and whereby the device automatically closes the supply circuit when normal temperature of the anode is restored.

9. The combination of an X-ray tube including a cathode and an anode, said anode normally operating at an elevated temperature which valies in response to variations of energy supplied to the tube, an energy supply circuit for said tube, a thermo-responsive safety device having a temperature-fiexure characteristic proportional to the heating and cooling characteristic of said anode, and means for adjusting said characteristic of said device, said device being positioned out of heat-conducting relation with respect to the anode, means including said energy supply circuit for heating said safety device during operation with energy which varies in proportion to the energy supplied to said device whereby the temperature of the safety device follows closely the temperature variations of said anode, and means whereby the safety device opens said energy supply circuit when the anode reaches an 10 WILLIAM K. KEARSLEY. 

